The use of natural products as starting materials for the manufacture of various large-scale chemical and fuel products which are presently made from petroleum- or fossil fuel-based starting materials, or for the manufacture of biobased equivalents or analogs thereto, has been an area of increasing importance.
Agricultural raw materials such as starch, cellulose, sucrose or inulin are inexpensive and renewable starting materials for the manufacture of hexoses, such as glucose and fructose. It has long been appreciated in turn that glucose and other hexoses, in particular fructose, may be converted into other useful materials, such as 2-hydroxymethyl-5-furfuraldehyde, also known as 5-hydroxymethylfurfural or simply hydroxymethylfurfural (HMF):
The sheer abundance of biomass carbohydrates available provides a strong renewable resource base for the development of commodity chemical and fuel products based on HMF.
In view of this fact, and due to HMF's various functionalities, it has been proposed that the HMF thus obtainable from hexoses such as fructose and glucose, could be utilized to produce a wide range of products derived from renewable resources, such as polymers, solvents, surfactants, pharmaceuticals, and plant protection agents. HMF has in this regard been proposed, as either a starting material or intermediate, in the synthesis of a wide variety of compounds, such as furfuryl dialcohols, dialdehydes, esters, ethers, halides and carboxylic acids.
A number of the products discussed in the literature derive from the oxidation of HMF. Included are hydroxymethylfurancarboxylic acid (HmFCA), formylfurancarboxylic acid (FFCA), 2,5-furandicarboxylic acid (FDCA, also known as dehydromucic acid), and diformylfuran (DFF). Of these, FDCA has been discussed as a biobased, renewable substitute in the production of such multi-megaton polyester polymers as poly(ethylene terephthalate) or poly(butylene terephthalate). Derivatives such as FDCA can be made from 2,5-dihydroxymethylfuran and 2,5-bis(hydroxymethyl)tetrahydrofuran and used to make polyester polymers. FDCA esters have also recently been evaluated as replacements for phthalate plasticizers for PVC, see, e.g., WO 2011/023491A1 and WO 2011/023590A1, both assigned to Evonik Oxeno GmbH, as well as R. D. Sanderson et al., Journal of Appl. Pol. Sci. 1994, vol. 53, pp. 1785-1793.
While FDCA and its derivatives have attracted a great deal of recent commercial interest, with FDCA being identified, for instance, by the United States Department of Energy in a 2004 study as one of 12 priority chemicals for establishing the “green” chemical industry of the future, the potential of FDCA (due to its structural similarity to terephthalic acid) to be used in making polyesters has been recognized at least as early as 1946, see GB 621,971 to Drewitt et al, “Improvements in Polymer”.
Unfortunately, while HMF and its oxidation-based derivatives such as FDCA have thus long been considered as promising biobased starting materials, intermediates and final products for a variety of applications, viable commercial-scale processes have proven elusive. Acid-based dehydration methods have long been known for making HMF, being used at least as of 1895 to prepare HMF from levulose (Dull, Chem. Ztg., 19, 216) and from sucrose (Kiermayer, Chem. Ztg., 19, 1003). However, these initial syntheses were not practical methods for producing HMF due to low conversion of the starting material to product. Inexpensive inorganic acids such as H2SO4, H3PO4, and HCl have been used, but these are used in solution and are difficult to recycle. In order to avoid the regeneration and disposal problems, solid sulfonic acid catalysts have also been used. The solid acid resins have not proven entirely successful as alternatives, however, because of the formation of deactivating humin polymers on the surface of the resins. Still other acid-catalyzed methods for forming HMF from hexose carbohydrates are described in Zhao et al., Science, Jun. 15, 2007, No. 316, pp. 1597-1600 and in Bicker et al., Green Chemistry, 2003, no. 5, pp. 280-284. In Zhao et al., hexoses are treated with a metal salt such as chromium (II) chloride in the presence of an ionic liquid, at 100 degrees Celsius for three hours to result in a 70% yield of HMF, whereas in Bicker et al., sugars are dehydrocyclized to HMF at nearly 70% reported selectivity by the action of sub- or super-critical acetone and a sulfuric acid catalyst.
In the acid-based dehydration methods, additional complications arise from the rehydration of HMF, which yields by-products such as, levulinic and formic acids. Another unwanted side reaction includes the polymerization of HMF and/or fructose resulting in humin polymers, which are solid waste products and act as catalyst poisons where solid acid resin catalysts are employed, as just mentioned.
In consideration of these difficulties and in further consideration of previous efforts toward a commercially viable process for making HMF, Sanborn et al. in US Published Patent Application 2009/0156841A1 (Sanborn et al) describe a method for producing “substantially pure” HMF by heating a carbohydrate starting material (preferably fructose) in a solvent in a column, continuously flowing the heated carbohydrate and solvent through a solid phase catalyst (preferably an acidic ion exchange resin) and using differences in the elution rates of HMF and the other constituents of the product mixture to recover a “substantially pure” HMF product, where “substantially pure” is described as meaning a purity of about 70% or greater, optionally about 80% or greater, or about 90% or greater. An alternative method for producing HMF esters performs the conversion in the presence of an organic acid, which can also serve as the solvent. Acetic acid is mentioned in particular as a solvent for fructose. The resulting acetylated HMF product is reported to be “more stable” than HMF, because upon heating HMF is described as decomposing and producing byproducts “that are not easily isolated or removed,” page 4, paragraph 0048. Sanborn et al. then proceed to describe the oxidation of the acetylated HMF product to provide FDCA, using a Co/Mn/Br Mid-Century type catalyst as well-known for the liquid phase oxidation of p-xylene to yield terephthalic acid (the conventional petroleum feed-derived commodity chemical that FDCA has been proposed to replace, for producing polyesters).
Still other recent publications describe like efforts to convert HMF to FDCA by a Mid-Century type oxidation process, see, for example, WO 2011/043661 to Muñoz de Diego et al. and WO 2013/033058 to Zuo et al, the latter involving a spray oxidation process with a solubilized Co/Mn/Br Mid-Century catalyst with acetic acid as a solvent.
While Mid-Century type oxidation methods are familiar to those in the terephthalic acid and polyester businesses and seemingly match up well with methods of making HMF derivatives as disclosed by Sanborn et al. and in the WO'661 reference to Muñoz de Diego et al., nevertheless these processes share the common challenge and added expense associated with removing and recycling an additional solvent such as acetic acid.
In WO 2013/106136 to Sanborn et al. (hereinafter “WO'136”, such application hereby being incorporated herein by reference), a process is described for making HMF from an aqueous hexose solution through an acid dehydration, without the necessity of proceeding with an added solvent and without the necessity of proceeding through a “more stable” HMF derivative in order to avoid the formation of humins and like materials that would be undesirable in the context of further oxidizing the HMF (or HMF derivative) to FDCA of a suitable purity for polymer applications. In the WO'136 application, more particularly, an aqueous hexose solution is rapidly heated from an ambient to a reaction temperature in the presence of an acid catalyst, with a limited per-pass conversion of the hexose sugars to HMF prior to separation of a fermentation-ready residual sugars product from the HMF product. By carrying out a rapid dehydration of the hexose sugars in this fashion, the overall exposure of the HMF that is formed to acidic, elevated temperature conditions is correspondingly limited. The residual sugars product can be recycled for conversion to additional HMF, can be fermented to produce various products or put to other uses as referenced in the WO'136 application.
In view of the WO'136 process, it would be advantageous if an effective catalyst and process were then available for oxidizing HMF in an aqueous HMF feed such as produced according to the WO'136 application (wherein acetic acid need not be used to produce an HMF ester derivative) to provide FDCA; omitting the acetic acid would enable the considerable expense of recovering and recycling the acetic acid to the dehydration step to be avoided. Unfortunately, however, the knowledge base for HMF oxidation using other than the familiar Mid-Century type catalysts is relatively more limited.
U.S. Pat. No. 8,193,382 to Lilga et al., for example, describes oxidizing HMF in an aqueous solution with air or oxygen using a heterogeneous supported platinum catalyst. The aqueous HMF starting material can be basic, neutral or acidic, though the “relatively low solubility” of HMF oxidation products such as FDCA in neutral and acidic water requires that “appropriate reactor designs” are utilized to accommodate solids formation or that the HMF concentration in the feed is limited to avoid generating FDCA concentrations over the solubility limit in a combination of acetic acid and water; in the alternative, it is said that “solids formation and feed concentration are typically not problematic” under “mildly basic conditions” generating a carboxylate salt of FDCA (col. 5, lines 1-13). Use of a strong base such as NaOH is however cautioned against, as possibly leading to “undesirable side reactions such as the Cannizzaro reaction” (col. 5, lines 13-15). The oxidation catalysts are prepared by a complex method which involves calcination, mixing with platinum (II) acetylacetonate, rotary evaporation, repeated calcination, and reduction under hydrogen to an activated product, and then passivation under a flow of 2% O2.
Casanova et al. “Biomass into Chemicals: Aerobic Oxidation of 5-Hydroxymethyl-2-furfural into 2,5-Furandicarboxylic Acid with Gold Nanoparticle Catalysts”, ChemSusChem, vol. 2, issue 12, pp 1138-1144 (2009) describe heterogeneous, nanoparticulate gold catalysts on a ceria support for the aerobic oxidation of HMF to FDCA, wherein NaOH is used at an optimal NaOH/HMF ratio of 4:1 to desorb the acid product from the catalyst surface.
Zope et al., “Influence of Reaction Conditions on Diacid Formation During Au-Catalyzed Oxidation of Glycerol and hydroxymethylfurfural”, Topics in Catalysis, vol. 55, pp 24-32 (2012) similarly evaluates the oxidation of an aqueous HMF solution in the presence of supported gold catalysts. An acknowledged “serious limitation” is that an added base is required, which increases the operating costs of the process and produces additional salt byproducts that have little value and may have a negative environmental impact (p. 25). Use of a highly basic catalyst support in place of the added inorganic base (NaOH) had been suggested by others, and while Zope et al. found that the product diacid (FDCA) was in fact formed with high selectivity, “extensive dissolution” of magnesium from the highly basic hydrotalcite support was also noted.
JP Patent Application Publication 2009-23916 differs somewhat in employing cupric bromide, 2,2′-bipyridine and (2,2,6,6,-tetramethylpiperidin-1-yl)oxyl (TEMPO) in an alkaline aqueous HMF solution to produce FDCA carboxylate salts by use of both the Cannizzaro reaction and oxidation. From 5.0 to 20 equivalents of alkali are indicated as most preferred for each molar equivalent of HMF, so that while the use of a precious metal catalyst is avoided, the process still does have the same “serious limitation” ascribed by Zope et al. to the processes using such precious metal catalysts.
Gorbanev et al., “Effect of Support in Heterogeneous Ruthenium Catalysts Used for the Selective Aerobic Oxidation of HMF in Water”, Topics in Catalysis, vol. 54, pp 1318-1324 (2011) prescribe heterogeneous ruthenium-based catalysts for the aerobic oxidation of HMF in water, ostensibly without the added base described as a “serious limitation” by Zope et al., though on closer examination it is evident that the heterogeneous ruthenium-based catalysts require a basic support (so that the catalyst preparation method in 2.2 describes the addition of a base, except in the case of MgO—La2O3, which gives a basic reaction mixture without a prior base treatment); a control experiment performed without a basic support led to only 3% of FDCA and 2% of HMFCA with the rest being formic acid. Moreover, consistent with Zope's findings, evidence of dissolution of the basic supports was noted (p. 1321, second paragraph). As well, consistent with Lilga et al's teachings, Gorbanev et al. utilized a dilute HMF feed concentration (0.6 percent by weight) and undesirably high catalyst to substrate ratios for their Ru-based catalysts.